In the process of slaughtering animals for food, considerable interest has developed in the electrical stimulation of the freshly slaughtered carcasses. It has been shown that this electrical stimulation produces a number of advantageous effects, including an improved tenderization of the carcass meat, a brighter more youthful lean color of the meat and less "heat-ring" formation. This electrical stimulation has been discussed in a number of publications; see "The National Provisioner" for June 4, 1977, page 11 et seq., "Meat Industry" for April, 1977, pages 32 and 77 and "Meat Industry" for July, 1978, page 38 et seq. The electrical stimulation is achieved by applying a plurality of pulses of a relatively high voltage electric current through the carcass. The voltages employed may be in the range of 550 volts and upward.
In commercial operations such an electrical stimulation process presents potential shock hazards to employees. Because of the voltages involved, an employee can potentially receive a shock which will cause severe injury if not death. The electrical systems employed are ungrounded, but as the apparatus is employed in a meat packing house there is a potential for the high voltage electrical system to become accidentlly grounded and as a result a worker, who necessarily also is grounded, can receive an electric shock from the system. It is likely that high humidity conditions exist in the packing plant and this may result in an accidental grounding. Of course, various other possibilities exist for an accidental grounding of the system.
The principal object of the present invention is to detect the existence of any such unintentional and accidental grounding and to shut down the system in the event that it occurs.
Ground fault interrupters are known and are commonly used in electrical systems for the protection against shock hazards. They are necessarily employed with systems in which one side of the electrical power is intentionally grounded. Their purpose is to shut off the flow of electrical power in the event that a person's body completes a circuit between the other side of the electrical power and ground, the shutting off of the power being conducted with such rapidity that the person will not be injured. The fact that they necessarily are employed with a grounded system prevents their use in a totally ungrounded system, such as that of an electric carcass stimulator. Furthermore they have an inherent problem in that if the wires carrying the electric power out from the ground fault interrupter are of significant length, and thus present significant capacitance, there will be a power loss which will cause a false or unintended tripping of the interrupter.
I have devised a method and apparatus for utilizing a ground fault interrupter to detect an accidental and inadvertent grounding of a normally ungrounded high voltage system, with the power into the system being shut off upon such grounding being detected. With one side of the input of the interrupter being grounded, I connect the other side of the output of the interrupter to the high voltage system through a rectifier and, generally, a smoothing filter so that only direct current can flow from the interrupter through the high voltage system to any accidental ground that may occur. When an accidental ground does occur, the interrupter sees more current flowing out than is returning (because of the direct current flow to ground) and, as in the normal functioning of such an interrupter, that condition causes the interrupter to open the circuit. Having made that detection of the accidental grounding, I then use the circuit-opening characteristic of the interrupter to open a switch in the high voltage system thereby removing the high voltage from that system. As in the subsequently described embodiment, this can be accomplished by having a normally open switch in the high voltage system, which switch can be closed only when electrical power is present at the output of the interrupter.
In the present invention, the sensitivity of the detection is adjustable by employing a variable resistor between the high voltage system and ground. This resistor functions to drain some direct current from the system to ground. Of course, if too much direct current is drained to ground through the resistor this will cause the interrupter to open its electrical circuit. But by setting the amount of drain to a value somewhat less than that required to trip the interrupter, the detection system can be made quite sensitive or, if desired, somewhat less sensitive.
Other objects and advantages will be apparent from the following description.